1. Field of the Invention
The present invention relates to a projection display that uses a light modulating device.
2. Description of the Related Art
There are projection displays that use a light modulating device such as a liquid crystal display (LCD) or a digital micromirror device (DMD) to modulate, in accordance with image data, light radiated from a light source, project the modulated light onto a screen, and display an image.
For example, a projection display using a DMD such as described in JP-A No. 2003-102030 expresses shades of gray of image information by switching the inclination of the mirrors of the DMD—that is, the reflectance on the screen—in accordance with the image information. The reflectance of the DMD on the screen takes two values: an ON state where the mirrors reflect light onto the screen and an OFF state where the mirrors do not reflect light onto the screen. The light projected onto the screen in the ON state becomes the maximum luminance that can be expressed as an image. In the OFF state, ideally light does not reach the screen if the reflectance is zero, but in actuality some light reaches the screen even in the OFF state. This becomes the minimum luminance that can be expressed on the screen. In projection displays at present, the maximum luminance and the contrast ratio that is the ratio between the maximum luminance and the minimum luminance are both definitely not sufficient in terms of display performance, and projection displays that have a larger maximum luminance and a larger contrast ratio are desired.
In order to raise the maximum luminance, there is a method that increases the amount of light of the light source, but when the amount of light of the light source is increased, the maximum luminance rises but the minimum luminance also rises at the same time, so the contrast ratio is not improved. Further, the power consumption of the light source also becomes larger and, as a result, there is the problem that the lifespan of the light source becomes shorter. Thus, simply increasing the amount of light of the light source is not a good solution.
In regard to the contrast ratio, as described in JP-A No. 5-66501, for example, there is a method that improves the contrast ratio by adjusting the amount of light inputted to the light modulating device. That is, the method improves the contrast ratio by raising the maximum luminance by increasing the amount of light inputted to the light modulating device when the screen is to be bright, and conversely lowering the minimum luminance by decreasing the amount of light inputted to the light modulating device when the screen is to be dark.
Further, as described in JP-A No. 2002-107662, there is a method where an iris diaphragm is interposed between the light source and the light modulating device. An iris diaphragm changes the light transmission amount by mechanically changing the size of the aperture to change the area through which light passes. An iris diaphragm has the advantage that it is capable of dimming about 25 to 100% so its dimming range is relatively wide. However, the increase in cost resulting from the iris diaphragm and a motor for driving the iris diaphragm has become a large problem. Further, although this method should be capable of dimming 0 to 100% because it mechanically adjusts the aperture, uniformity in the luminance on the screen becomes poor when the area through which the light passes becomes equal to or less than a certain value, so there has been an inherent limit on the dimming range.
Moreover, in projection displays of this type, a high intensity discharge (HID) lamp such as a metal halide lamp or an ultra high pressure mercury lamp is usually used as the light source. Tungsten is used for the electrodes in an HID lamp, and it is well known that a halogen cycle is utilized in order to alleviate wear of the tungsten resulting from the heat of arc discharge.
A halogen cycle is the circulation cycle of an appropriate amount of halogen gas entrapped inside the bulb of an HID lamp. Now, when the temperature of the arc spot of the tungsten electrodes exceeds the sublimation heat of tungsten, the tungsten vaporizes and tungsten atoms are released inside the bulb. The released tungsten atoms bond with the halogen atoms inside the bulb to form halogenated tungsten and float inside the bulb. If the bulb inner wall is an appropriate temperature, then the halogenated tungsten circulates by convection and returns to the electrodes without adhering to the bulb inner wall. The halogenated tungsten returning to the electrodes is separated by heat so that the tungsten atoms return to the electrodes and the halogen atoms float inside the bulb. The recurrence of this reaction is called a halogen cycle.
Because of this cycle, wear of the tungsten electrodes is alleviated and blackening resulting from tungsten adhering to the bulb inner wall is controlled. Incidentally, when the temperature of the bulb inner wall is too low, the halogenated tungsten adheres to the bulb inner wall, and when the temperature of the bulb inner wall is too high, the halogenated tungsten separates at the bulb inner wall and the tungsten atoms adhere to the bulb inner wall. For that reason, it is necessary to maintain the temperature of the bulb inner wall within an appropriate temperature range in order for the halogen cycle to be appropriately performed.
In HID lamps that utilize the halogen cycle in this manner, there is the problem that when the input power is adjusted in order to control the amount of light of the lamp, the temperature of the bulb inner wall also changes in response to the power, and when the temperature of the bulb inner wall is outside the appropriate range, the halogen cycle cannot be performed appropriately and the lifespan of the lamp becomes shorter. Consequently, the adjustable power range becomes limited when the temperature of the inner wall of the bulb is set in the appropriate range.
Through an experiment by the inventors in the present case, it was found that the adjustable range of power in an ultra high pressure mercury lamp with a rating of 150 W is 110 to 190 W when the temperature of the bulb is set in the temperature range that lamp manufacturers prescribe as a condition for use. Because the amount of light of a lamp is virtually dependent on power, the dimming range is 60 to 100%. With a dimming range of this extent, the effect of improving the contrast ratio is small, so there are few advantages to be put to practical use with this method alone.